The present invention relates to a hydraulic auxiliary-force control, in particular hydraulic auxiliary-force steering (power-assisted steering) for motor vehicles, comprising a shaft part on the manipulating side, which is allocated to the conventional steering wheel, as well as a shaft part coaxial thereto on the control-gear side, a servo-valve unit arranged on one shaft part and having a displaceable control spool, a control member adjusting the control spool as a function of relative rotations between the shaft parts, and a spring unit which is effective between the shaft parts and attempts to press the shaft parts as well as the control member into a center position relative to one another.
A hydraulic auxiliary-force steering is described in German Examined Patent Application 1,924,032. In this publication, the spring unit between the shaft parts is formed by a torsion bar which is accommodated in longitudinal bores of the shaft parts. At each of its ends, the torsion bar is connected to one of the shaft parts in such a way as to be fixed in terms of rotation, the connection being effected by pins which pass through radial bores, in alignment with one another, in the shaft parts as well as the allocated ends of the torsion bar. The manufacturing effort associated therewith is comparatively great.
In addition, it is disadvantageous in this known auxiliary-force steering arrangement that the torsion bar yields elastically even at small torques effective between the shaft parts, and in fact even when the torsion bar is dimensioned to be rigid. The result of this is that the control spool of the servo valve is pushed away from its center position even at small transmitted torques. Thus a more or less large auxiliary force is always effective. This is often undesirable, particularly in auxiliary-force steering systems, since the roadway condition or the driving circumstances have hardly any influence on the subjective driving feel on the steering wheel.
European Patent Specification 0,190,960 describes a hydraulic auxiliary-force steering arrangement in which a shaft part on the manipulating side and a shaft part coaxial thereto on the control-gear side are elastically coupled to one another in such a way that, during relative rotations of the two shaft parts, an annular part arranged between them is axially displaced. This axial stroke is transmitted by a lever mechanism to the spool of a servo valve.
This arrangement is comparatively expensive. In addition, the spool of the servo valve here is displaced even at relatively small torques effective between the shaft parts. This is because even when the spring unit is dimensioned to be rigid, it yields even in the face of small forces, even if only slightly.
In an auxiliary-force steering arrangement disclosed in German Unexamined Patent Application 3,545,858, a lever acting as a regulating member for the valve spool of the servo valve is arranged between two coaxial shaft parts. This lever is pivotably articulated on one shaft part, or a part firmly connected thereto, about an eccentric axis approximately parallel to the shaft axis as well as on the other shaft part, (or part firmly connected thereto). The lever is at a radial distance from the eccentric axis as well as from the shaft axis and, with one lever end, actuates the control spool. Effective between the regulating lever and one shaft part is a spring unit which attempts to press the regulating lever into a center position relative to the one shaft part. Accordingly, the regulating lever assumes a double function. On the one hand, it serves to actuate the control spool and, on the other hand, it serves, together with the spring unit, to produce a restoring force which forces the shaft parts as well as the regulating lever into a normal position relative to one another. In addition, this regulating lever also limits the possible relative rotation between the shaft parts by its interaction with stops arranged on one shaft part.
This known auxiliary-force control is distinguished by a small space requirement and relatively small demands on the capacity of the pressure source of the auxiliary-force steering. Furthermore, the spring unit is designed in such a way that it can have more or less high preloading and accordingly only permits a relative rotation of the shaft parts at a minimum torque between the same so that an auxiliary force is produced only when the minimum torque is exceeded. Nonetheless, a constructive simplification of this known auxiliary-force control is desirable.
An object of the present invention is to provide an auxiliary-force control in which the effort involved in assembly is relatively slight, while at the same time making it possible for an auxiliary force to be produced only when a minimum torque is reached between the shaft parts.
This and other objects are achieved by the present invention which provides a hydraulic auxiliary-force steering control for motor vehicles comprising a shaft having a first shaft part on a manipulating side, a second shaft part coaxial to the first shaft part on a control-gear side, and a shaft axis. A servo-valve unit is arranged on one of the first or second shaft parts and has a displaceable control spool. A regulating member is coupled to the control spool and adjusts the control spool as a function of relative rotations between the first and second shaft parts. A spring unit between the first and second shaft parts is biased to press the first and second shaft parts and the regulating member into a center position relative to one another. The spring unit comprises a leg spring with legs and a helical spring area connecting the legs. A central recess and first radial slots with shoulders are provided in one of the first or second shaft parts. The central recess accommodates the helical spring area and the shoulders of said first radial slots serves as seating for the legs. Second radial slots with walls are provided in the other of the first or second shaft parts, these legs extending into the second radial slots. The legs of the leg spring bear against at least one of the walls of the second radial slots and against at least one of the shoulders of the first radial slots when the first and second shaft parts are in the center position relative to one another.
On account of the arrangement and design according to the invention, the leg spring, during assembly of the auxiliary-force control, can be inserted into the said recess on one shaft part or on the part firmly connected thereto. The leg spring maintains this position on account of its preloading, since it is firmly secured in the recess by frictional connection with walls of the recess or allocated slots. Thus the leg spring at the same time assumes a position in which the two shaft parts can be assembled virtually free from force in their center position relative to one another.
In addition, it is advantageous that the leg spring is effective directly between the shaft parts and is arranged separately from the control member. By this functional separation, the assembly of regulating member and spring unit is additionally facilitated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.